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Ubiquity of interplanetary nanodust particles from Cassini observations

11 April 2015

An international team, led by astronomers from Paris Observatory, has measured for the first time the distribution of nanodust particles in the interplanetary medium between the orbits of Earth and Jupiter. This result confirms that these particles are released near the Sun and ejected outwards at several hundreds of kilometers per second, as predicted by theoretical models.

These nanoparticles bridge the gap between molecular ions and dust grains. They were detected for the first time in the solar wind at 1 Astronomical Unit from the Sun (1 AU is the Sun-Earth distance) by the STEREO probes in 2009. Their small size gives them peculiar properties. Since a large proportion of their atoms are lying at their surface, they interact in a privileged way with their environment. And their electric charge is high enough for their dynamics to be governed by electromagnetic forces. In this way, nanodust particles can be accelerated by the magnetized solar wind and expelled from the inner heliosphere to large distances at nearly 400 km/s.

When such a fast nanoparticle hits a spacecraft, the impact produces a microcrater whose material is vaporized and ionized, producing an expanding plasma cloud (Figure 1).

Figure 1. Principle of the detection of fast dust by a radio instrument on board a spaceship (credit : Meyer-Vernet et al. 2010, DOI : 10.1063/1.3395912)
Figure 1. Principle of the detection of fast dust by a radio instrument on board a spaceship (credit : Meyer-Vernet et al. 2010, DOI : 10.1063/1.3395912)

Figure 1. Basics of the detection of fast dust particles with a wave instrument onboard a space probe. (credit: Meyer-Vernet et al. 2010, DOI : 10.1063/1.3395912)

The plasma electric charges produce a transient signal that can be detected by a radio receiver connected to electric antennas (Figure 2).

Figure 2. Electric field impulsions measured by an electrical antenna of the instrument RPWS on board Cassini at about 1.5 astronomical unit from the sun (credit : Schippers et al 2015, ApJ in press)
Figure 2. Electric field impulsions measured by an electrical antenna of the instrument RPWS on board Cassini at about 1.5 astronomical unit from the sun (credit : Schippers et al 2015, ApJ in press)

Figure 2. Electric pulses measured by an electric antenna of the RPWS instrument on the Cassini spacecraft at about 1.5 AU from the Sun (credit : Schippers et al 2015, ApJ in press).

The analysis of the spectral power density of the voltage measured at several distances from the Sun has revealed how the nanodust flux decreases with heliocentric distance (Figure 3).

Figure 3. Flux of nano dust particles measured by the instrument RPWS/HFR on board Cassini between the orbits of Earth and Jupiter. By comparaison, the color symbols near 5 AU represent measures previously made near Jupiter (credit : Schippers et al 2015, ApJ in press)
Figure 3. Flux of nano dust particles measured by the instrument RPWS/HFR on board Cassini between the orbits of Earth and Jupiter. By comparaison, the color symbols near 5 AU represent measures previously made near Jupiter (credit : Schippers et al 2015, ApJ in press)

Figure 3. Nanodust flux measured by the RPWS/HFR radio receiver onboard Cassini between the orbits of Earth and Jupiter. For comparison purposes, the color symbols near 5 AU show previous measurements near Jupiter (credit: Schippers et al 2015, ApJ in press).

This measured flux is in agreement with the release of nanodust particles near the Sun and their acceleration to speeds as high as 400 km/s by the electric field of the magnetized solar wind, as predicted by models of nanodust dynamics. Far enough from Jupiter, these nanoparticles originating in the inner heliosphere outnumber largely those ejected by this planet.

Credits: The Principal Investigator of the RPWS instrument is W. S. Kurth, who replaced D. Gurnett in 2014 (Université d’Iowa, USA). The receiver RPWS/HFR was designed and built in LESIA (Observatoire de Paris, INSU-CNRS, Universities Paris 6 and Paris 7) with the support of CNES and CNRS.

References

  • Schippers P., Meyer-Vernet N., Lecacheux, A., Belheouane, S., Moncuquet, M., Mann, I., Kurth W. S., Mitchell D. G., André, N. : 2015, Nanodust detection between 1 and 5 AU by using Cassini wave measurements, Astrophys. J., in press,
  • Schippers P., Meyer-Vernet N., Lecacheux, A., Kurth W. S., Mitchell D. G., André, N. : 2014) Nanodust detection near 1 AU from spectral analysis of Cassini/Radio and Plasma Wave Science data, Geophysical Research Letters, 41, 5382-5388, DOI: 10.1002/2014GL060566

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